1. Field of the Invention
The present invention relates to a scanning microscope using a particle beam. More specifically, the present invention relates to an optimum inspection apparatus using a particle beam for performing observation or inspection of a fine dimension and/or an appearance structure of a semiconductor device.
2. Description of the Invention
A known critical dimension evaluation apparatus using an electron beam as a particle beam acquiring a sample image based on secondary electrons by scanning an electron beam on a semiconductor sample and inspecting a dimension concerning the characteristic pattern for the sample is disclosed in Japanese Patent Application Number 60-161514. A known appearance inspection electron beam apparatus inspecting an abnormality of a sample by comparing an image of a scanning electron microscope with a standard pattern is disclosed in Japanese Patent Application Number 5-258703. A method introduced by these apparatus has high resolution for the image compared to an inspection method using an optical source and is very useful for inspecting a sub-micron semiconductor pattern.
In the prior art apparatus, an optimum focus adjustment of an electron lens system is performed based upon signals from at least 3 sample images obtained by scanning the electron beam on the same sample under different conditions of focusing shift of the electron lens system. In order to reduce the time spent for focus adjustment, a focus adjustment means for detecting a height of a sample using a height sensor using light reflection (Z sensor) and adjusting the focus of an electron lens system based upon a table produced in advance to reduce the deviation of a height of a sample from the standard is indicated in a Japanese patent application number 8-273575. Since there are such problems that a sample surface measured by the Z sensor does not completely accord with an extreme surface of the sample and that has a disadvantage in a reappearance of the focus adjustment due to hysteresis of magnetization in the electron lens system, resolution of the sample image decreased without the focus adjustment using an electron beam scanning.
A method for obtaining a high resolution for a sample image based upon mathematical conversion (integral conversion) of the sample image using a beam profile measured in advance is introduced in a Japanese patent application number 2-181639. However, this method does not consider variation of the beam profile due to variation of the focusing-shift for each spot from which the sample image is taken.